Recovery of titanium from titanium bearing materials

ABSTRACT

A method of recovering titanium dioxide from a raw material additionally containing aluminium includes the steps of grinding the titanium dioxide raw material, reacting the particulate raw feed material with sulphuric acid under specified conditions, digesting and filtering the resultant cake material containing titanyl sulphate, if present, treating the solution to remove calcium and/or iron, precipitating out the aluminium as aluminium ammonium sulphate, hydrolysing the remaining titanyl sulphate solution and, after washing the hydrolysate, calcining the hydrolysate to produce titanium dioxide.

BACKGROUND OF THE INVENTION

[0001] THIS invention relates to the recovery of titanium from titaniumbearing materials, and in particular to a method of recovering titaniumdioxide or titanium metal from a titanium dioxide bearing material.

[0002] Highveld Steel and Vanadium Corporation is a large manufacturerof steel using its own unique steel manufacturing process. The slagproduced in this steel manufacturing process is rich in titaniumdioxide, typically in amounts of 22 to 32% of the slag material.

[0003] Pure titanium dioxide is white in color and is, therefore, avaluable pigment used in many applications such as the production ofpaints, paper, cement, polymers and the like. The slag produced in theHighveld Steel manufacturing process is an ideal source of titaniumdioxide for this purpose.

SUMMARY OF THE INVENTION

[0004] A method of recovering titanium dioxide from a raw materialcontaining the titanium dioxide and aluminium, typically as its oxide,comprises the steps of:

[0005] a) grinding the titanium dioxide bearing material to form aparticulate raw feed material;

[0006] b) contacting the particulate raw feed material with apredetermined amount of sulphuric acid in a reaction vessel and raisingthe temperature in the reaction vessel to a predetermined temperature atwhich a reaction takes place to produce a cake material containingtitanyl sulphate;

[0007] c) contacting the cake material with a sufficient quantity ofwater, and optionally recovered process acid, to dissolve the cakematerial, which contains the titanyl sulphate;

[0008] d) filtering the resultant suspension and collecting the solutioncontaining the titanyl sulphate and, if present, optionally treating thesolution to remove iron and/or calcium;

[0009] e) contacting the titanyl sulphate containing solution withammonium sulphate to produce aluminium ammonium sulphate in thesolution;

[0010] f) precipitating the aluminium ammonium sulphate out of thesolution and separating it from the titanyl sulphate containingsolution;

[0011] g) hydrolysing the titanyl sulphate containing solution bycontacting the solution with water, which has first been seeded with anappropriate amount of rutile and heated, or a portion of previouslyhydrolysed solution containing hydrated titanium dioxide, and heatingthe solution to boiling point to precipitate out hydrated titaniumdioxide;

[0012] h) washing the hydrolysate with an ammonium solution to removeresidual sulphates as ammonium sulphates followed by filtering off thehydrated titanium dioxide; or

[0013] i) filtering the hydrolysate followed by washing with sodiumhydroxide, ammonium hydroxide, water, phosphoric acid and/or dilutedsulphuric acid; and

[0014] j) calcining the hydrolysate to drive off any residual acid andwater of crystallisation to produce titanium dioxide.

[0015] The raw material typically also contains vanadium, calcium and/oriron typically as their oxides.

[0016] The vanadium is typically removed as VOSO₄ in the solutionremaining in step g).

[0017] In this regard, the Ti³⁺ (as TiO₂) in the hydrolytic solution ispreferably kept at a concentration of 3 to 4 g/l Ti³⁺ to preventhydrolysis of the VOSO₄ and incorporation into the hydrated TiO₂product.

[0018] The insoluble solid residue produced in step c) is preferablyseparated from the solution by clarification, typically with theaddition of gellatene and/or glue, dextrine, tannic acid, AMPAM or otherappropriate clarifier, before the suspension is filtered in step d).

[0019] The hydrolysis step g) is preferably carried out in the absenceof a prior crystallisation and vacuum concentration step having takenplace.

[0020] The cake material of step b) is preferably left to mature forabout 2 to 3 hours at a temperature of about 190° C. to 250° C. beforebeing dissolved in step c).

[0021] In step c), air is preferably introduced with the water, andoptionally recovered process acid, in order to assist with agitation todissolve the cake.

[0022] The air is preferably cold air to control the reactiontemperature below about 75° C., typically 70 to 75° C., in order toprevent premature crystallisation of TiO₂.

[0023] The slag in step a) is preferably ground to form a particulatematerial in which at least 80% of the particles are able to pass througha 45 micron mesh.

[0024] The reaction of the sulphuric acid solution and particulate feedmaterial in step b) typically takes place in a fusion reactor, which maybe a batch or continuous fusion reactor.

[0025] The temperature is preferably raised in the fusion reactor byintroducing pre-heated air into the reaction vessel.

[0026] After the desired amount of water has been introduced, air andmechanical agitation is used to break the cake into a homogenoussuspension.

[0027] The calcium is typically removed as calcium sulphate, typicallyduring the filtration step d).

BRIEF DESCRIPTION OF THE DRAWING

[0028] The invention will now be described in more detail, by way ofexample only, with reference to the accompanying drawing, which is aschematic flow diagram of a preferred embodiment of a method ofrecovering titanium dioxide according to the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0029] The present invention is directed at a so-called sulphatingprocess for recovering titanium, in particular titanium dioxide, from atitanium bearing material, in particular a slag produced in a steelmanufacturing process containing titanium dioxide and aluminium, andtypically also vanadium, calcium, and/or iron, typically as theiroxides.

[0030] The process of the invention, as set out in the accompanyingdrawing, is particularly suited for recovering TiO₂ from a slag as setout below. Total TiO₂    24-30% FeO     2-8% Fe₂O₃     2-8% CaO   14-16% MgO    12-15% SiO₂    20-25% Al₂O3    12-14% V₂O₅   0.2-1% MnO  0.2-1% S 0.01-0.2% Na₂O   0-0.2% P₂O₅    0.01% H₂O    0.06%

[0031] The titanium slag (elkem slag) available is dried as needed andground to a uniform fine particle size of less than 45 μm. Thepulverized slag is mixed with approximately 1.3 parts by weight of 93%to 98% sulphuric acid and stirred with compressed air in a reactorvessel. The mixture is heated with pre-heated air in the reactor vessel.At about 100° C. the exothermic reaction starts, increasing rapidly whenthe temperature reaches 180° C., and the slag is converted into a solidmass composed of soluble titanium, aluminium, vanadium and ironsulphates. The use of heated air instead of heated steam has been foundto be preferable. The reason for this is that the Highveld Steel slagused in the process has free iron present, in an amount of about 4%, inthe ferrous state. Accordingly, it is undesirable to have water in thefusion reaction as it would transform the free iron to the Fe⁺⁺⁺ state.In addition, the presence of oxygen in the heated air assists in therecovery of the titanium by converting Ti⁺⁺⁺ to Ti⁺⁺⁺⁺.

[0032] Cold air is then blown through the cake for about 4 hours afterthe reaction is completed to produce a porous cake. The cake so producedis left to mature at a temperature of about 190 to 250° C for about 2 to3 hours.

[0033] The reaction cake is then digested in water, and optionallyrecovered process acid, in a ratio of water to solid of about 3:1 inorder to dissolve the desired titanium compounds. During theintroduction of water, air is introduced simultaneously to assist withagitation, as mechanical agitation would not at this stage be effectivedue to the solid cake formed. During the introduction of air and wateran exothermic reaction takes place. As it is important for the reactiontemperature to be controlled below 75° C. in order to avoid prematurecrystallisation of the TiO₂, proper flow control of the cold air intothe reactor vessel is maintained. After the desired amount of water hasbeen introduced, air and mechanical agitation is used to break the cakeinto a homogeneous suspension. The appropriate time to achieve thesuspension is in the order of 4 hours, but visible inspection willindicate when this has been achieved.

[0034] Some of the titanium compounds which are believed to go intosolution when digesting the cake with water include:

TiO₂×H₂O

TiOSO₄.H₂O

TiOSO₄.2H₂O

TiSO₄.H₂SO₄.2H₂O

TiSO₄.H₂SO₄.H₂O

Ti(SO₄)₂.

[0035] Where ilmenite is used as a raw material, the solution typicallycontains trivalent or “ferric” iron. This is reduced to the divalent or“ferrous” form with scrap iron as reducing agent. This step is notrequired in the case of the Highveld Steel slag as the iron in the slagis already in the “ferrous” state.

[0036] Following leaching the insoluble solid residue is separated fromthe resulting solution by clarification. The elkem slag containsimpurities, some of which easily form colloidal solutions, such as SiO₂and Al₂O₃. Most of the hydrated silica is destroyed under the highreaction temperature. The insoluble solids can easily be separated fromthe leaching solution by clarification with the addition of gellateneand/or glue, dextrine and tannic acid. AMPAM clarificant solution canalso be used.

[0037] The suspension so obtained is filtered through a filter system,typically using large settling tanks. The filtrate consists mainly ofcalcium sulphate CaSO₄ and residue. The residue is then washed withwater for disposal. Alternatively, the CaSO₄ residue can be treated torecover H₂SO₄.

[0038] A very dense yellow solution is obtained after filtering, whichis rich in peroxide TiO₃.2H₂O. By way of information, the peroxide is anoxidising substance which is unstable in the presence of water and ismuch more soluble than TiO₂ or TiO₂.H₂O. It dissolves in acid solutionswith the formation of yellow to red pertitanyl ions (TiO₂ ⁺⁺). Inalkaline solutions, it forms titanate ions (HTiO₃ ⁻) and/or colourlesspertitanate ions (HTiO₄ ⁻ and TiO₄ ⁻). The solubility of the peroxide isaround 1 g.mol/l at a pH of 0.5 (acid medium) and at a pH of 12(alkaline medium). By the action of hydrogen peroxide on very acidsolutions of tri- or tetravalent titanium, a solution of peroxidizedTiO₂ ⁺⁺ ions is obtained, which deposits as a precipitate of peroxide,TiO₃.2H₂O, upon increasing the pH.

[0039] The clarified titanyl sulphate solution contains approximately100 gpl TiO₂ and 15 gpl Al₂(SO₁)₃.

[0040] If the solution is concentrated to about 200 gpl TiO₂, thealuminium sulphate will be precipitated as a crystalline solid at roomtemperature, and the solution will become an “unflowed solid”. Thereforethe aluminium sulphate must be removed from the clarified solutionbefore concentration. The ammonium sulphate is added to the solution,which then reacts with the aluminium sulphate in it as follows:

Al₂(SO₄)₃ +(NH₄)₂SO₄+12H₂O→NH₄Al(SO₄)₂.12H₂O

[0041] After reaction, the aluminium ammonium sulphate is precipitatedin crystalline solid form and separated from the solution.

[0042] The solution is then filtered to remove traces of insolublematerials

[0043] During the hydrolysis step, the titanium solution is transformedinto a white titanium oxyhydrate slurry. The steps that have gone beforeare fundamental in preparing the titanium compounds for hydrolysis. Onestep which is not required when using Highveld slag is crystallizationand vacuum concentration. Thus, hydrolysis is carried out by contactingthe titanyl sulphate containing solution with heated water which hasbeen seeded with nucleating or seeding agents, in particular nucleirutile, and then boiled.

[0044] The formation of titanyl hydroxide proceeds according to thefollowing reactions:

Ti⁺⁺⁺⁺+2OH⁻→TiO⁺⁺+H₂O and

TiO⁺⁺+2OH⁻→TiO(OH)₂

[0045] To increase the rate of thermal hydrolysis of sulphate solutionsat atmospheric pressure and at the same time obtain products of pigmentgrade, nucleating or seeding agents are added. Normally only 1% nucleior seed agent is required.

[0046] The composition, purity, and physical properties ofhydrolytically precipitated titanium dioxide depend to a large extentupon the conditions under which the decomposition takes place, such ascomposition of the solution employed, temperature, and duration ofboiling. In the commonly employed processes, large changes in theconcentration of the solution would take place as the hydrolysate isformed and an equivalent amount of acid is liberated. Thus the formationof titanic acid will take place under entirely different conditions atthe beginning and at the end of the operation.

[0047] To overcome this effect, a titanium-rich solution is preparedfrom the slag, transferred into a precipitation vessel and heated untilpractically complete hydrolysis has taken place. Four fifths of theliquor is then removed. To the remaining one fifth, still at theprecipitation temperature, fresh pregnant solution is added at such arate as to secure a practically constant concentration of dissolvedtitanium until the vessel is filled. Heating is continued throughout theprocess. The supply of solution is then interrupted, and four fifths ofthe liquor is again removed. The operation is repeated as often as isnecessary. The above process only requires initial introduction ofnuclei or seeding agents, thereafter the one fifth liquor containsenough nuclei seeding agents to initiate the hydrolysis reaction. Thetotal cycle time of the exercise is between 3 and 6 hours.

[0048] The titanyl sulphate solution treated in the above hydrolysisstep contains about 1% V₂O₅ and the weight ratio of vanadium to titanium(as V₂O₅/TiO₂) is about 1%. The behavior of titanyl sulphate andvanadium sulphate are different. If the pH value of the solution isabout 3, the vanadium sulphate will be hydrolyzed. However, if thehydrolytic solution has about 3 to 4 gpl Ti³⁺ (as TiO₂) and about 20% offree sulphuric acid, the vanadium sulphate does not hydrolyse.

[0049] Test results of hydrolysis showed that about 65% of the vanadiumin the hydrolytic solution remained in the mother solution, theremainder being precipitated with the hydrated TiO₂. During the washingprocess, the pH value of the washing water increases. The vanadiumsulphate absorbed into the surface of hydrated TiO₂ will be hydrolyzedand incorporated into the hydrated TiO₂. In order to prevent vanadiumfrom going into the product, the Ti³⁺ content of the hydrolytic solutionis controlled between 3 to 4 gpl (as TiO₂).

[0050] The TiO₂.xH₂O is removed by means of filtering through afiltering system. The hydrolysate is then washed with either sodiumhydroxide, ammonium hydroxide, water, phosphoric acid or dilutesulphuric acid to improve the properties of the titanium white.

[0051] The vanadium solution can be heated with a 25% NH₃ solution,crystallised and filtered to recover the vanadium and (NH₄)₂SO₄.

[0052] The main reason for washing the hydrolysate is to neutralize theliquor and to improve the crystal properties. Conditioning agents suchas dilute acids and zinc or aluminium powder or a powerful non-metallicreducing agent or phosphoric acid or an alkaline metal could also beintroduced during this washing stage, to ensure the formation of therutile structure during the calcination process that follows.

[0053] The thoroughly purified and washed hydrolysate obtained by thethermal hydrolysis of titanium salt solutions is an amorphous hydrousoxide which still contains impurities as chemi-adsorbed acid. Inaddition, it is too fine-grained and almost amorphous, which isundesirable for pigment grade TiO₂. In the production of pigment gradeTiO₂, accordingly, a calcination step is necessary to drive off thewater and residual acid and at the same time convert the titaniumdioxide to the crystalline form of a required particle size. At the sametime, desired pigmentary properties are developed.

[0054] Amorphous titanic oxide or hydroxide (TiO.xH₂O.SO₃), such as isobtained from the sulphate solution, is converted to the cryptocrystalline modification of pigment grade TiO₂ by calcination at 950° C.for 1 hour. The calcined TiO₂ is finally ground to 325 mesh powderhaving a purity of greater than 99.9%.

1. A method of recovering titanium dioxide from a raw materialcontaining the titanium dioxide and aluminium, comprises the steps of:a) grinding the titanium dioxide bearing material to form a particulateraw feed material; b) contacting the particulate raw feed material witha predetermined amount of sulphuric acid in a reaction vessel andraising the temperature in the reaction vessel to a predeterminedtemperature at which a reaction takes place to produce a cake materialcontaining titanyl sulphate; c) contacting the cake material with asufficient quantity of water, and optionally recovered process acid, todissolve the cake material, which contains the titanyl sulphate; d)filtering the resultant suspension and collecting the solutioncontaining the titanyl sulphate and, if present, optionally treating thesolution to remove iron and/or calcium; e) contacting the titanylsulphate containing solution with ammonium sulphate to produce aluminiumammonium sulphate in the solution; f) precipitating the aluminiumammonium sulphate out of the solution and separating it from the titanylsulphate containing solution; g) hydrolysing the titanyl sulphatecontaining solution by contacting the solution with water, which hasfirst been seeded with an appropriate amount of rutile and heated, or aportion of previously hydrolysed solution containing hydrated titaniumdioxide and heating the solution to boiling point to precipitate outhydrated titanium dioxide; h) washing the hydrolysate with an ammoniumsolution to remove residual sulphates as ammonium sulphates followed byfiltering off the hydrated titanium dioxide; or i) filtering thehydrolysate followed by washing with sodium hydroxide, ammoniumhydroxide, water, phosphoric acid and/or diluted sulphuric acid; and j)calcining the hydrolysate to drive off any residual acid and water ofcrystallisation to produce titanium dioxide.
 2. A process according toclaim 1, wherein the raw material contains iron and/or calcium, whichis/are removed from the titanyl sulphate solution in step d).
 3. Amethod according to claim 1 or claim 2, wherein the raw materialcontains vanadium, which is removed as VOSO₄ in the solution remainingin step g).
 4. A method according to claim 3, wherein the Ti³⁺ (as TiO₂)in the hydrolytic solution is kept at a concentration of 3 to 4 g/l Ti³⁺to prevent hydrolysis of the VOSO₄ and incorporation into the hydratedTiO₂ product.
 5. A process according to any one of claims 1 to 5,wherein the aluminium and the vanadium, calcium and iron, when present,are present as their oxides.
 6. A method according to claim 1, whereinthe insoluble solid residue produced in step c) is separated from thesolution by clarification, before the suspension is filtered in step d).7. A method according to claim 6, wherein the clarification step iscarried out with the addition of gellatene and/or glue, dextrine, tannicacid, AMPAM or other appropriate clarifier.
 8. A method according toclaim 1, wherein the hydrolysis step g) is carried out in the absence ofa prior crystallisation and vacuum concentration step having takenplace.
 9. A method according to claim 1, wherein the cake material ofstep b) is left to mature for about 2 to 3 hours at a temperature ofabout 190° C. to 250° C. before being dissolved in step c).
 10. A methodaccording to claim 1, wherein in step c), air is introduced with thewater, and optionally recovered process acid, in order to assist withagitation to dissolve the cake.
 11. A method according to claim 10,wherein the air is cold air to control the reaction temperature belowabout 75° C., in order to prevent premature crystallisation of TiO₂. 12.A method according to claim 11, wherein the reaction temperature iscontrolled between 70 and 75° C.